Known types of indoor lighting arrangements can comprise various different types of light source such as halogen lamps, incandescent lamps, etc. Such lamps may be used in conjunction with various kinds of dimmers, for example phase-cut dimmers that cut a portion of the mains input power in order to reduce the average power provided to the conventional light source. LED lighting—using light-emitting diodes as light sources—is also becoming more widespread and popular owing to the reduced power consumption and longer lamp lifetimes. LED lamps are available with standard connectors such as E14, E27, GU10 etc., so that these lamps may be used to retrofit existing lighting fixtures. WO2013177167A1 discloses method and apparatus for controlling a lighting device wherein a dimming characteristic in an energy source is detected and a switch is controlled based on the dimming characteristic to transfer energy to the load.
However, such retrofit LED lamps should not be connected to the output of a legacy phase-cut dimmer such as a wall-mounted dimmer, since damage to the LED lamp is likely to occur. The range of a phase-cut dimmer can extend from a minimum (undimmed, corresponding to 100% or maximum light output) up to a maximum (fully dimmed, corresponding to minimum light output), and the “phase-cut angle” defines how much of the 180° half-wave is cut by a leading-edge or trailing-edge dimmer. At a minimum dimming setting, the phase-cut angle can be close to 0°, and at a maximum dimming setting, the phase-cut angle can approach 180°. The average voltage at the input to the lamp is decreased as the phase-cut angle increases. In the case of an LED lamp, this phase-cut voltage is applied to the inputs of the driver, and the reduced voltage can be a problem for the LED driver, particularly at large phase-cut angles. To warn the user, the manufacturer generally provides a printed warning with the lamp. However, this cannot reliably prevent the incorrect installation of the LED lamp.
For some types of LED lamp, it will not be immediately obvious to the user that the lamp has been connected to a legacy dimmer, since the lamp driver will manage to deliver a constant current to the LEDs until a certain maximum phase-cut angle is reached. Until this maximum phase-cut angle is reached, the light output remains essentially constant. Beyond this maximum phase-cut angle, the driver will not be able to match the input power to the required output, and the lamp will exhibit unstable behaviour, for example by noticeably flickering or by ceasing to emit light.
However, even below the maximum phase-cut angle, when the driver can ensure that the light output is constant, the behaviour of the lamp can still be compromised. For example, the sharp voltage increase on a leading-edge phase-cut input can result in a current spike in the lamp circuitry. Furthermore, ripple current can increase significantly in various parts of the lamp circuitry. The current spikes and/or ripple current can ultimately lead to lamp damage, but the effects are not noticeable to the user until the LED lamp starts to behave poorly or fails well before its expected lifetime.
Newer generations of LED lamps, in which the lamp driver has a high power factor, can exhibit a reaction to the dimmer setting of a legacy dimmer, i.e. the light output of LED lamp will be affected by the phase-cut angle. However, these LED lamps are also not intended to be used with a phase-cut dimmer. The user may misinterpret the apparent “response” of the lamp to mean that the lamp is dimmer-compatible, and may even interpret the “response” to actively use the dimmer to adjust the light output. As a result, the LED lamp may show erratic behaviour and may fail well before its expected lifetime.
Since the user may be unaware that an older or newer generation LED lamp has been incorrectly installed in an existing lighting setup, in spite of the manufacturer's printed warning, customer dissatisfaction in the event of premature lamp failure or unsatisfactory behaviour may be directed instead at the lamp and the lamp manufacturer.
Therefore, it is an object of the invention to provide an improved way of incorporating LED lamps into existing lighting arrangements, avoiding the problems described above.